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Old 09-22-2013, 09:51 AM   #1
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Walking a twin-screw boat sideways

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Originally Posted by Art View Post
Regarding Steering a Twin Screw:

To properly know how to handle a forward or reverse moving twin screw boat without using helm-wheel steering, via alternating engines’ rpm and shift positions only, it simply takes experience/practice.

In difference to single screw (although with a single screw bow and stern thrusters play well at very slow speeds - i.e. docking, near standstill) twin screw can be maneuvered quite well in forward or reverse via engines alone at nearly any speed... without using helm-wheel steering... as long as rudders are maintained in straight position during the forward or reverse travel. Rudders, as well as per engine alternate rpm and shift positions do come into play on twins when moving boat directly sideways from a standstill, while not trying to simultaneously attempt forward or reverse travel – see #3.

1. At higher speeds the rpm allowed per each engine can alter course as needed as well as maintain the course needed... even for long periods of forward travel if necessary.
2. At very low rpm (docking, slow canal travel and the like) carefully actuated per engine shift positions in coordination with their rpm can keep boat placed where desired or maneuvered into positions desired.
3. For making a standstill twin screw boat to move directly sideways to starboard or port (without thrusters)... the following works well:

“Directly-Sideways” Movement/Handling of Twin Screw Boat

Move boat sideways to port (opposite items for starboard)

- Turn rudders 80% +/- to starboard
- Place starboard in forward and port in reverse
- Starboard kept at idle rpm / Port approx 150 rpm higher

Once rudders in position with engines in the correct gear at idle rpm quickly adjust direction desired engine rpm - higher equivalent rpm for both engines with same %age difference = faster sideways motion – to a point – it can get hairy, go slow! Correction can become a bitch if boat gets moving sideways at too quick a speed... especially in close quarters! Also, depending on rudder size and prop size the numbers mentioned may need to be adjusted. However, the general mechanical/physics properties of thrust and water flow remain the same for described sideways travel.

Take it slow: Gently move a boat laterally. Be careful to not get boat moving too quickly as stopping sideways motion takes considerably more rpm, shift, and steerage adjustment time than simply forward or reverse or circular rotation motion adjustments. Adjust rudder and shift/throttle controls as needed for current and wind conditions to move latterly away from dock. Practice makes perfect. I recommend practice in a completely open area... at least at first!

Thanks for posting this (in another thread), but I have to admit our results aren't yet promising. Perhaps I just wasn't able to overcome the wind, when I was trying it...

I do assume "turn rudders ... to starboard" means putting the wheel to starboard... which would in turn cause a turn to starboard if going ahead...

-Chris
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Old 09-22-2013, 10:41 AM   #2
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Lock the wheel over to the opposite direction you wish to move the boat. This method even works even on my 36' Nova with a 4' keel and lots of windage.
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Old 09-22-2013, 11:03 AM   #3
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Don't expect to overcome much wind or current. This will vary from boat to boat, and is related to the amount of rudder exposed to your prop wash.

You are using prop wash on the rudder from the engine in forward to push the stern over, and this sideways force must be greater than the sideways force created by the engine in reverse walking the stern in the opposite direction. With the amount of throttle roughly balanced, if your rudders are too small,or not "hard over", that driving force will not be enough, and when you increase power on that side, you will have an excess of forward motion,that will need to be counteracted with additional reverse on the other side.

It is a sometimes tough balancing act, which is always easier in calm conditions.

When you get practiced, folks on the dock will look for your "thrusters" and will wonder out loud why your thrusters don't make the usual rude growling noises they have become used to hearing.
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Old 09-22-2013, 12:25 PM   #4
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If you really want to walk a bigger twin perfectly sideways with negligible fore or aft movement, get a bow thruster. With big winds and currents it really helps.
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Old 09-22-2013, 12:56 PM   #5
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Quote:
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If you really want to walk a bigger twin perfectly sideways with negligible fore or aft movement, get a bow thruster. With big winds and currents it really helps.
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Old 09-22-2013, 01:06 PM   #6
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Quote:
Originally Posted by koliver View Post
Don't expect to overcome much wind or current. This will vary from boat to boat, and is related to the amount of rudder exposed to your prop wash.

You are using prop wash on the rudder from the engine in forward to push the stern over, and this sideways force must be greater than the sideways force created by the engine in reverse walking the stern in the opposite direction. With the amount of throttle roughly balanced, if your rudders are too small,or not "hard over", that driving force will not be enough, and when you increase power on that side, you will have an excess of forward motion,that will need to be counteracted with additional reverse on the other side.

It is a sometimes tough balancing act, which is always easier in calm conditions.

When you get practiced, folks on the dock will look for your "thrusters" and will wonder out loud why your thrusters don't make the usual rude growling noises they have become used to hearing.

Yeah, I've always understood the theory... but I suspect we've usually bailed early when it didn't seem to work and just used a spring line to warp in...

And we've got small-ish rudders, at least in the grand scheme of things.

Yesterday I decided to really practice, but the situation was windy... so I'll just have to wait for another opportunity...

Certainly a thruster -- or two -- wouldn't be turned down, but oddly enough, I don't see a line of folks queuing up to pay for that

-Chris
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Old 09-22-2013, 01:40 PM   #7
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(Chris-- Sent you a PM on this theory).
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Old 09-22-2013, 02:35 PM   #8
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This system definetly does work ..... on some boats !
Hull form, including keel length and depth, propeller pitch, rudder size, maximum rudder angle all make a difference. Throw in some windage, wind and current and it complicates matters quickly.
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Old 09-22-2013, 02:46 PM   #9
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(Chris-- Sent you a PM on this theory).

Umm... I'm a little puzzled why private? And then why announce it?

-Chris
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Old 09-22-2013, 02:51 PM   #10
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Umm... I'm a little puzzled why private? And then why announce it?

-Chris
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My Cruisers does a nice job when it's not blowing.
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Old 09-22-2013, 05:38 PM   #11
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My Cruisers does a nice job when it's not blowing.
It is actually the ghost of Marin...
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Old 09-22-2013, 07:06 PM   #12
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I used to walk our 24' express cruiser sideways, involved a lot of wheel spinning & shifting.
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Old 09-22-2013, 07:45 PM   #13
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If I got a thruster I wouldn't be able to face the "old salts" in our club who say: "thrusters are for people who can't handle a boat."
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Old 09-22-2013, 07:56 PM   #14
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Quote:
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If I got a thruster I wouldn't be able to face the "old salts" in our club who say: "thrusters are for people who can't handle a boat."
I've never been one to try and impress others..... To each his own though.
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Old 09-22-2013, 08:17 PM   #15
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Like others have said, different boats react to this differently. I've been able to do this a couple of times facing into the current and not fighting too much crosswind. Remove the head current or add a bit of crosswind and all bets are off.
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Old 09-22-2013, 10:57 PM   #16
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Quote:
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If I got a thruster I wouldn't be able to face the "old salts" in our club who say: "thrusters are for people who can't handle a boat."
Tell the old codgers to go pound sand... they probably once said the same thing about that newfangled black box that gives position from space.

The thing that counts is that you know your boat and are proficient docking... using any tools at hand. I used to bring Volunteer in without using the bow thruster when the conditions were right and the Admiral was not aboard.. she considered it stupid not to use it as it made for a higher percentage of successful landings ( and I didnt want to suffer the harassment of the quiet smirk she gives as I backed out for a missed approach ).

Our new Ocean Alexander is a single and doesn't have a thruster, I figured I would install one this spring but as time goes by I find the boat handles great without one. So far in the thirty or so dockings since August I am 30/30... somewhere there is a major bungled landing waiting to bust my bubble.

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Old 09-22-2013, 11:16 PM   #17
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Old 09-22-2013, 11:39 PM   #18
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If I got a thruster I wouldn't be able to face the "old salts" in our club who say: "thrusters are for people who can't handle a boat."
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Old 09-22-2013, 11:44 PM   #19
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Old 09-23-2013, 06:19 AM   #20
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Usually some fwd momentum helps. For a flank (walk) to port, come in slow with a little bit of fwd creep, turn the helm hard to STBD. as the turn starts to STBD, back the port engine and leave the stbd engine in fwd. It takes practice and there is a balancing act of helm and clutches. BE Patient! give the vsl time to react. YMMV
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